Particulate emissions from an engine can be reduced with a particulate filter trap and a regeneration system to periodically clean the filter trap of particulates by incineration. Generally, durable and acceptable filter particulate traps have been developed by the art which have included wire mesh (see U.S. Pat. No. 3,499,269) and, more advantageously, rigid ceramics, perferably in a honeycomb monolithic cellular wall structure (see U.S. Pat. Nos. 4,276,071; 4,329,162; and 4,340,403).
Ceramic monolithic honeycomb celled filter traps have shown 60-80% particulate collection efficiency for applications in diesel powered passenger cars and light and heavy duty trucks. The collection of particulates in the filter trap results in an increasing exhaust gas back pressure with mileage accumulation. After a relatively short driving period, which depends on the filter trap volume and particulate level entrained in the exhaust gas flow, the filter trap will require regeneration to minimize the loss in fuel economy and performance associated with the increased exhaust gas back pressure. Regeneration is accomplished by raising the temperature of the particulates on the inlet face of the filter trap to approximately 1200.degree. F. using a fuel fed burner or electrical heating system.
Previously published schemes used to initiate regeneration have all used a manually operated trigger which, of course, can lead to inadequate regeneration, the operator failing to initiate the regeneration system precisely when it is needed. One attempt to provide an automatic initiation system is disclosed in copending U.S. Application Ser. No. 463,687, invented by the inventors herein and assigned to the assignee herein. Such system uses an on-board computer system together with a differential pressure sensor. The computer memory contains an entire map of the clean trap back pressure as a function of engine speed, load (fuel delivery), and exhaust temperature. A differential pressure sensor is used to provide the actual instantaneous pressure drop across the trap. This instantaneous trap pressure drop is compared with the clean trap pressure drop at the instantaneous engine speed, load, and exhaust temperature. If the trap pressure drop is greater than the specified multiple of the clean trap pressure drop, then regeneration is automatically initiated.
This system is complex and expensive because it requires a memory of clean trap pressures at various speed, fuel delivery, and exhaust temperature combinations. It would be of significant technical help if the need for an on-board computer could be eliminated while still providing for an automatic initiation of the regenerative apparatus according to the needs of the filter trap.